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Old 07-30-2018, 02:20 PM   #26
gmachinz
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Re: 1973-1991 Dual fuel tank systems theory of operation

A lot of times its just the simple things. The fuel sender has an arm attached to the float which makes contact with the resistance board as full levels move up and down. Throughout the full motion you get anywhere from 0-96 ohms on average. Most of the time if you have more than 50K miles on a stock sending unit, that resistance board will be worn enough that the gauge is off noticeably. Most worn units I’ve seen is from half tank and less-most people dont keep their tanks topped off while never dipping below a half tank for example. The high mileage resistance boards are always worn out at half tank and less which is why the gauge reads fine on full but near a half tank the gauge tends to wander all over the place. Quality of the sending units are worse with offshore replacements than GM or NOS ones as well.
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Old 08-01-2018, 11:27 AM   #27
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Re: 1973-1991 Dual fuel tank systems theory of operation

will you please respond to my pm thanks!
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Old 08-01-2018, 01:06 PM   #28
gmachinz
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Re: 1973-1991 Dual fuel tank systems theory of operation

PM sent again.
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Old 08-03-2018, 10:46 AM   #29
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Originally Posted by 808_67C10 View Post
HA! So, I started the truck this morning and the gas gauge read right on full.... then, after driving about 2-3 miles, she topped out about 1/4" past Full - I swear, these square bodies and their electrical issues. I miss my 67 c10 so bad right now! LOL
3:00 on the gauge is usually your ground from the sender ring to the frame... It can be a damaged wire or dodgy connection between the sender and the gauge but usually it's the short little ground wire.

Try grounding the fuel tank to the frame with some jumper cables. If that fixes it the ground wire is your 3:00 needle problem...
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1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005.5 VW Jetta BRM PD-TDI, 02J 5 Speed
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Old 08-03-2018, 07:12 PM   #30
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Originally Posted by hatzie View Post
3:00 on the gauge is usually your ground from the sender ring to the frame... It can be a damaged wire or dodgy connection between the sender and the gauge but usually it's the short little ground wire.

Try grounding the fuel tank to the frame with some jumper cables. If that fixes it the ground wire is your 3:00 needle problem...
That is what it was... HATZIE - you are amazing! Thank you!
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Old 08-07-2018, 06:37 AM   #31
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Re: 1973-1991 Dual fuel tank systems theory of operation

Lots of great info here. Thanks
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Old 08-09-2018, 09:44 PM   #32
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Re: 1973-1991 Dual fuel tank systems theory of operation

91 tbi truck, took out fuel pumps, and hard tubed it for a diesel swap. I'm getting power to the switching valve, but I think I've lost the ground due to unplugging the fuel pumps, am I correct? whats the simplest way to convert the tbi setup for a diesel setup? do I just need to ground the two wires that were originally power for the fuel pump? or do I need to hook the two together? factory wiring diagrams leave out alot!
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Old 08-10-2018, 12:02 AM   #33
hatzie
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Re: 1973-1991 Dual fuel tank systems theory of operation

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Originally Posted by v30crewcab View Post
91 tbi truck, took out fuel pumps, and hard tubed it for a diesel swap. I'm getting power to the switching valve, but I think I've lost the ground due to unplugging the fuel pumps, am I correct? whats the simplest way to convert the tbi setup for a diesel setup? do I just need to ground the two wires that were originally power for the fuel pump? or do I need to hook the two together? factory wiring diagrams leave out alot!
NO!! You haven't lost the ground... The valve motor wires from the dash switch change polarity depending on the switch position. The dash switch is a polarity reversing switch commonly used to control the direction of DC motors. Go back and read the 81-91 section in post #2 of this thread. Pay close attention to the power routing diagrams I drew up.

The only wiring modifications you need to make are at the two position Weatherpak plugs attached to the pigtails dangling from each sender. DO NOT DO ANYTHING WITH THE TWO POSITION PLUGS ON THE ENDS OF THE NL2 VEHICLE HARNESS.

Remove the fuel pump power wire from the Weatherpak plugs in each sender pigtail and replace it with a green Weatherpak cavity plug. Fold over the fuel pump wire and tuck it into the split poly loom around the sender pigtail.
You only need to mod the sender pigtail. AGAIN!!! DO NOT DO ANYTHING WITH THE TWO POSITION PLUGS ON THE ENDS OF THE NL2 VEHICLE HARNESS. If you ground these wires you'll have a fire because either could be hot depending on the dash switch position.
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1959 M35A2 LDT465-1D
1967 Dodge W200 B383, NP420/NP201
1969 Dodge Polara 500 B383, A833
1972 Ford F250 FE390, NP435/NP205
1976 Chevy K20, 6.5L, NV4500/NP208
1986 M1008 CUCV
2000 GMC C2500, TD6.5L, NV4500
2005.5 VW Jetta BRM PD-TDI, 02J 5 Speed
2009 Impala SS LS4 V8


RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials...Please include at least the year and model in your threads. It'll be easier to answer your questions.
And please let us know if and how your repairs were successful.
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Old 08-10-2018, 09:00 AM   #34
v30crewcab
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Re: 1973-1991 Dual fuel tank systems theory of operation

ok, so it should still be switching without the fuel pumps in the loop? I've got power and ground at the switch, but the valve is not switching, even with a new switching valve. I'll have to investigate more. Thanks.
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